Remove "enabled" output from show_index_cache_command

show_index_cache_command prints whether the index-cache is enabled.
This text was added back in 2018 in commit 87d6a7aa (Add DWARF index
cache).  Then in 2021, the enabling option was changed via commit
7bc5c369 (gdb: introduce "set index-cache enabled", deprecate "set
index-cache on/off").

This latter change made this output, IMO, redundant.  That is,
currently gdb will show:

    (gdb) show index-cache
    ...
    index-cache enabled:  The index cache is off.
    ...
    The index cache is currently disabled.

This patch removes the redundant output.